1. Field of the Invention
The present invention relates to a transparent conductive film used for a transparent electrode of a display, such as a liquid crystal display, an electroluminescence display, and an electrochemichromic display, an image sensor, a solar cell or the like.
2. Description of the Related Art
Various transparent conductive films have been produced by forming a thin film of, e.g., metal, such as gold and platinum, or oxide, such as tin oxide, indium oxide and mixture thereof, on a substrate. An indium-tin-oxide (ITO) film, which comprises indium oxide and tin oxide added thereto, is widely used for various devices, particularly, a liquid crystal display, because the ITO has superior properties such as a high transparency, a lower resistivity, a larger etch rate, a better chemical stability, and a stronger adherence to a substrate, among known transparent conductive films.
To lower the resistivity of the transparent conductive film of oxide materials, the oxide materials are made semiconducting materials, due to a control of (1) the valence thereof or (2) a reduction thereof.
The resistivity-lowering technique based on the valence control semiconductorizing system is used in the formation of the ITO film or the following film formations.
According to Japanese Unexamined Patent Publication (Kokai, JP-A-) No. 59-163707, at least one of the oxides of ruthenium oxide, lead oxide, and copper oxide is added to ITO to obtain a transparent conductive film having an excellent resistivity of 0.7.times.10.sup.-4 .OMEGA.-cm (ohm-centimeter) and a superior light transmittance of 88%. According to Japanese Unexamined Patent Publication (Kokai, JP-A-) No. 59-71205, 1.01 to 3 wt % of phosphorous oxide is added to ITO to obtain a transparent conductive film 100 nm thick and having an excellent resistance of 0.3 .OMEGA./.quadrature. (resistivity of 3.times.10.sup.-4 .OMEGA.-cm) and a superior light transmittance of 90%. According to Japanese Unexamined Patent Publication (Kokai, JP-A-) No. 61-294703, aluminum fluoride is added to indium oxide to obtain a transparent conductive film 60 nm thick and having an excellent resistance of 220 .OMEGA./.quadrature. (resistivity of 13.times.10.sup.-4 .OMEGA.-cm) and a superior light transmittance of 85%. Furthermore, according to Japanese Unexamined Patent Publication (Kokai, JP-A-) No. 63-78404, aluminum fluoride is added to ITO to obtain a transparent conductive film 150 nm thick and having an excellent resistance of 5 .OMEGA./.quadrature. (resistivity of 0.75.times.10.sup.-4 .OMEGA.-cm) and a superior light transmittance of 84%.
The resistivity-lowering technique based on the reaction control semiconductorizing system is used in the formation of a transparent conductive film of the Zelez et al. patent (U.S. Pat. No. 4,399,194). According to the Zelez et al. patent, the transparent conductive film comprising 40 to 60 wt % (60.0 to 7.2 mol %) of zirconium oxide and the remainder of indium oxide has a light transmittance of 83% and a resistivity of 4.4.times.10.sup.-4 .OMEGA.-cm, after annealing (see Table II, column 4).
The methods of forming a transparent conductive film include physical vapor deposition (PVD) methods such as vacuum deposition, ion-plating and sputtering, chemical reactive deposition (CVD) methods such as thermal decomposition, and coating layer deposition (CLD) methods such as spraying and dipping. Among these methods, the PVD methods (specially, the sputtering method) are widely used, as a transparent conductive film having a good density and a low resistivity can be easily formed thereby.
Currently, liquid crystal displays are made larger for word processors, television receivers or the like, which require an improvement of the light transmittance of a transparent conductive film without a conventional level of an electric resistance thereof. Furthermore, a thickness of the transparent conductive films is larger than 200 nm, and these films require a relatively long etching time. In this case, an undesirable break of patterned lines formed on the film, and/or a nonuniformity of the resistivity of the film in the display due to a deterioration of the film surface conditions occur, to lower a production yield of the displays. If the electric resistivity of the films can be lowered, the thickness of the films can be reduced, with the result that such a film can be etched with decreasing the above-mentioned disadvantages.